(1) Field of the Invention
This invention relates to a metallic grid structure for a suspended ceiling, and more particularly to a connector in such structure that aids in keeping the ceiling intact, and without sagging from its own weight, during a fire.
(2) The Prior Art
A typical prior art suspended ceiling 10 is shown in FIG. 1 of the present drawings, wherein, in a grid 20, main beams 21 extend parallel to one another, at, for instance, 4′ intervals. Cross beams 31 in the grid 20 span the distance between the main beams 21 and interlock with the main beams 21 through slots 28 in the main beams 21.
In the grid 20, main beams 21 are connected end to end in a variety of ways, including the one shown in U.S. Pat. No. 6,523,313, incorporated herein by reference. Cross beams generally are connected to the main beams through slots in the main beams 21, as for instance, by a hook in a connector, as shown, for instance, in U.S. Pat. No. 6,199,343, incorporated herein by reference. The present invention relates to a hook type connector.
The grid 20 is suspended from a structural ceiling by hang wires, and ceiling panels 33 are laid in the rectangular openings formed by the grid 20 and supported on the flanges of the beams 21 and 31 to form the suspended ceiling 10. The panels 33 are generally of a highly heat resistant type. The suspended ceiling 10 extends in a flat plane in its normal established position.
In the event of a fire, the metallic beams 21 and 31 seek to expand longitudinally from stresses created in the beams 21 and 31 by the elevated temperatures. Unless a beam is allowed to expand longitudinally to relieve the stresses, the beam will buckle, permitting a panel 33 to fall out of the opening, and allowing the heat of the fire to reach and destroy the structural ceiling. Thus, it is important to keep the suspended ceiling 10, with its highly heat resistant panels 33, intact, so that the suspended ceiling acts as a heat shield for the structural ceiling.
Additionally, during a fire, the suspended ceiling 10, including the beams and panels, tends to weaken from the heat of the fire, and sag or droop from the normally flat plane of the ceiling.
Such tendency to droop of, for instance, a cross beam, places a withdrawal or pullback force on the cross beam connector, notwithstanding that the cross beam must expand longitudinally to relieve the stresses induced by the heat of the fire to avoid buckling. Thus, it is also important to keep the ceiling from drooping or sagging to prevent the panels from dropping out of the grid, thus creating another condition, in addition to buckling beams, wherein the structural ceiling is exposed to the heat of the fire.
Once the fire subsides, the ceiling 10 is generally replaced, since even though the ceiling desirably survives the fire relatively intact to provide such heat shield to the structural ceiling, visually and structurally the ceiling is generally impaired.
Numerous means exist to permit the beams to expand longitudinally in a controlled manner so they continue to support the panels in the event of a fire. Such means for expansion are divided into those for main beams, and those for cross beams.
A representative main beam expansion means is shown, for instance, in U.S. Pat. No. 6,351,919, incorporated herein by reference. These expansion means consist essentially of cutouts along the beam that fold under compressive forces, while keeping the beam longitudinally aligned.
Representative cross beam expansion means are shown, for instance, in the '343 patent, and also in U.S. Pat. No. 5,839,246, incorporated herein by reference. Such expansion means consist essentially of tabs on the connectors on the ends of the cross beams, that break away and permit the connector to further enter into the slot, thus permitting the cross beam to stay longitudinally aligned to continue to support the panel.